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Gears/Transcript
Transcript Text reads: The Mysteries of Life with Tim and Moby Tim rides his bicycle. He is wearing a helmet. He huffs and puffs loudly, out of breath. TIM: I'm in worse shape than I thought. He looks behind him. Moby is holding on to the back of Tim's bike. He has wheels on his feet and is wearing a bike helmet of his own. TIM: Why, you little— Tim turns to confront Moby, and Tim and his bike tip and fall to the ground. Moby remains standing. Tim reads from a typed letter. TIM: Dear Tim and Moby, what are gears and what do they do? From, Stan. A gear is a device that transfers motion from one part of a machine to another. Most gears are wheels or cylinders. These are sometimes called cogs. An animation shows two gears, a large one and a small one, meshed together and rotating. TIM: Just about every machine with moving parts, from clocks to cars to DVD players, uses gears. Images represent the machines Tim mentions. TIM: Gears usually have teeth on their edges that mesh together. When one of them turns, it forces the other gear to turn with it. An animation shows a close-up of two gears meshing as they turn. TIM: Gears can also be connected by a chain, like on this bike. You often see little gears paired up with bigger gears. An image shows a bicycle chain that runs from the large gear on the pedal to the small gear on the back wheel. TIM: One reason to do that is to increase torque, or turning power. MOBY: Beep. TIM: Well, big gears turn with more power than small gears. The larger the radius, the more force a turning wheel has. Gears take advantage of that difference. Images show a rear view of two people on a paddleboat and a paddle wheeler boat moving up a river. TIM: Check out this electric car window. Like many machines, it's driven by a small motor, which doesn't use up a lot of electricity. These little guys spin really fast, but without much torque. Definitely not enough force to move that heavy glass up and down. An animation shows an electric car window going up and down. The animation includes the workings of the window inside the car door and the window's motor. Tim easily stops the spinning motor with his fingers, and it begins to smoke. TIM: To increase the torque, you attach the motor to a gear train, a set of two or more gears. The gear connected directly to the motor will be small, and the next gear of the train will be bigger. In this gear train, the smaller gear has twelve teeth, and the bigger one has thirty-six. That means the small gear has to turn three times to turn the big gear once. An animation shows a very small gear meshing with a very large one in a gear train. An equation shows 36 ÷12 = 3. TIM: So we say that this train has a 1:3 gear ratio. MOBY: Beep. TIM: Right. The big gear turns one-third as fast as the little one, but with a lot more torque. So you're trading in rotational speed for force. Gear trains set up this way create a mechanical advantage, magnifying the amount of force put into them. Think of it this way. You could stop a car window's motor with your fingers, but not the window itself. Side-by-side animations show Tim stopping a car window's motor with his fingers and Tim pinching his fingers in a car window while trying to make the window stop. TIM: You can add more gears to the train to get higher and higher gear ratios and more output force. An animation shows several interconnected rotating gears. MOBY: Beep. TIM: Oh, yeah. Gear ratios work the other way too, trading in force to gain speed. Like in an electric fan, you want as much rotational speed as possible. An animation shows a spinning wall fan. TIM: To do that, you set up a gear train with progressively smaller gears. The animation shows the gears in the center of the fan: a small gear meshing with three medium-sized gears, which mesh with one large gear. TIM: On a bicycle, your legs are the motor. You use a big gear in front and a small gear in back to get more speed. And a small gear in front paired with a big gear in the back for more torque. An animation shows Tim riding a bicycle downhill, then uphill. MOBY: Beep. TIM: Well, gears can do lots of other things, too. When two gears are directly engaged, what do you notice about them? MOBY: Beep. TIM: Right. They spin in opposite directions. An animation shows two meshing, rotating gears. TIM: Any time the direction of a mechanical motion needs to be reversed, you can just add another gear to it. A third gear appears, meshing and rotating in the opposite direction as the gear next to it. TIM: That's useful for any machine that has to be able to move in two directions, like cars or electric drills. And not all gears are round, by the way. These specially shaped bevel gears are used to change the axis of a machine's rotation. An image shows two meshing bevel gears. They intersect at approximately a 90º angle. TIM: That's especially useful in cars, where you have to translate the motion from a shaft, spinning down the middle, to the wheels. An animation illustrates how the crankshaft of a car transfers motion from the engine to the two back wheels using bevel gears. TIM: And this worm gear rotates once for every single tooth on the cog it's engaged with. That's useful for getting a really high gear ratio. MOBY: Beep. An image shows a worm gear with spiral-like teeth. TIM: Yeah, maybe I could use a worm gear on my bike. Or a little less dead robot weight. Moby pedals the bike while Tim holds onto his back. Category:BrainPOP Transcripts